ELECTRONIC INTERACTIONS AND INTERCHROMOPHORE EXCITATION TRANSFER

The nature and variation of the electronic interactions which promote the transfer of singlet electronic excitation between two chromophores are elucidated using a simple four-electron model as a foundation. Two important distance regimes for excited-state intermolecular interactions can be distinguished: short range (ca. 3-6 Angstrom) and intermediate range (ca. 6-20 Angstrom). A unifying model is presented which accounts for both interchromophore orbital overlap effects, which dominate at short-range (using a variational treatment) and intermediate-range interactions (by a quantum electrodynamical description). It is shown that penetration as well as exchange effects are important at close separations. The magnitudes of higher multipole terms at intermediate separations are examined, demonstrating that for dipole-allowed (but weak) electronic transitions dipole-octopole and octopole-octopole contributions can be significant. The relevance of the results to excimer formation, molecular exciton interactions as well as interchromophore electronic energy transfer is described. The distance dependence of interchromophore singlet-singlet electronic energy transfer is addressed, highlighting the marked increase in rate with increasing interchromophore orbital overlap.